A host of water-soluble acrylic copolymers have been proposed. Such copolymers have been proposed for use in a multitude of products. It is known that the molecular weight, molecular distribution, the type of copolymerized monomers and their amounts will have a substantial affect upon the acrylic copolymer properties. It is also known that the manner in which these acrylic copolymers are prepared will affect its ultimate character, properties and functionality. The catalyst system, nature of the polymerization reaction (e.g., solvent, emulsion, etc.), dispersants and solvent system, reaction temperature, presence or absence of chain terminators, etc. are factors which bear upon the acrylic copolymer properties.
The interior of metal containers (steel, tin, aluminum, etc.) and their closures such as caps and lids are conventionally coated with resinous materials to protect the contained products from metal contamination. These metal interiors are typically coated (e.g., brushing, spraying, dipping, rollercoating, etc.) with a thermosetting formulation which when baked provides a water-resistant, solvent-resistant, thermoset coating. These thermoset coatings must meet stringent standards to qualify for such an end-use. During the coating application stage, the thermosetting formulation must readily adhere and uniformly coat the metallic container part. The formulation should lend itself to use in high-speed can coating operations. Non-uniformity or incomplete or non-adherence of the coating upon the metallic surface will frustrate its coating functionality.
The resin should also be capable of converting quickly and easily to an inert, thermoset, protective internal coating. The cured coating should possess a high degree of resistance towards physical and chemical degradation. It must also be sufficiently inert to protect the contained product from deterioration under such adverse conditions frequently encountered during its storage and shipment in commerce. Certain canned beverages (e.g., beer, soft drinks, etc) are reportedly susceptible to adulteration (e.g., adverse development of flavor, color, etc.), by extractable trace adulterants from the cured coating. Can coatings are frequently evaluated on the basis of blush resistance, adhesion, turbidity and fracture tests (e.g., see U.S. Pat. No. 3,219,729) as well as their resistance towards water, organic solvents, foods, chemicals, etc.
Organic solvent-based, epoxy-urea coatings have been used to interiorly coat cans (e.g., see U.S. Pat. No. 3,219,729). These can coatings reportedly have sufficient adhesion and flexibility so as to permit fabrication of coated sheets into can bodies, can ends, jar lids, bottle caps and other formed container components. These can coatings rely upon volatile organic solvents as a vehicle. Due to environmental, health and safety considerations, these organic based coating compositions have increasingly become subject to a greater degree of regulation and scrutiny by governmental bodies.
Attempts have been made to replace organic solvent-based, internal can coatings with water-based, thermosetting, coating systems. U.S. Pat. Nos. 3,996,182 and 3,957,709 disclose water-based systems which are reportedly useful as internal can coatings. These attempts have not been completely successful. Water-borne, internal can coating systems which preserve the taste, flavor and over-all quality of contained beverages while complying with the FDA and air pollution requirements would be a desirable goal. Such a system would provide an effective alternative to the existing organic-solvent-based coating system.